1. Field of the Invention
This invention relates generally to a gas sensor for detecting the presence of hydrogen in gas streams or in the environment, and for measuring the amount of such hydrogen. More particularly, in order to accomplish such detection the invention utilizes thin film technology, as well as a methodology for sensing the presence of hydrogen gas in such gas streams or environments.
2. Description of the Related Art
Hydrogen is a flammable and explosive gas with a wide variety of industrial and scientific uses. Well-known industrial uses of hydrogen include the production of basic staple products of chemical industry such as ammonia and fertilizers derived therefrom, basic alcohols, hydrogen chloride, reduction of ores for manufacturing of metals, refinery of oil for manufacturing of petroleum, hydrogenation of vegetable oils for margarine and related industries, and many other uses.
Hydrogen is also widely used for space flight applications, for instance as a component of hydrogen-oxygen blends used in vehicular propulsion systems.
Hydrogen is also used in a variety of metal forming and microelectronic processing steps which are often of extreme importance in device fabrication and metal interconnect processing of multi-level devices.
There has been also an increasing emphasis on the use of fuel cells, which require hydrogen as a fuel in various stationary and mobile applications, for instance, in fuel cells of automobiles.
It is axiomatic that handling hydrogen requires utilization of robust safety devices as it is a highly flammable gas at a concentration in air as low as 4% by volume. The ability to detect stray emissions of hydrogen is, therefore, mandatory, and is an important feature of any process or device where hydrogen is used.
In these and other applications, hydrogen sensors are employed to monitor the environment around which hydrogen is utilized, to ensure the efficiency, safety and operational integrity of the system. For such purposes, a number of hydrogen sensors and complex detection methods have been developed and are in common use.
A variety of these commercially available hydrogen sensors are based on measuring an electrical characteristic across a sensor element and at least four major categories of sensors and associated methods can be identified.
One type of hydrogen sensor is the xe2x80x9ccatalytic combustiblexe2x80x9d or xe2x80x9chot wirexe2x80x9d sensor (CC sensor) mentioned in the U.S. Pat. No. 6,006,582 to Bhandari, et. al. The CC sensor comprises two specially arranged beads of a catalytic metal or alloy, such as platinum-iridium wire heated to 600-800xc2x0 C. One bead is coated with a reactive catalyst. In the presence of a flammable gas, the heat of oxidation raises the temperature of the bead and alters the electrical resistance characteristics of the measuring circuit. This resistance change is related to the concentration of all flammable gases, including hydrogen, in the vicinity of the sensor.
The CC sensor has serious drawbacks. In oxygen deficient environments or above an upper explosive limit, the oxidation process is quenched causing difficulties in measuring. In addition, since the CC sensor is based upon oxidation, virtually any and all hydrocarbons have the same response as hydrogen, making it difficult to detect hydrogen in the environments which also contain hydrocarbons. Finally, the CC sensor element is easily contaminated by halogenated hydrocarbons and is susceptible to poisoning by silicones, lead and phosphorous.
Another commonly used hydrogen sensor is a non-porous metal oxide (MO) sensor. The MO sensor element comprises a non-porous metal oxide (such as zirconium dioxide or tin dioxide) sandwiched between two porous metal electrodes. Such electrodes are typically made of platinum. One electrode is exposed to the reference gas, usually, air, and the otherxe2x80x94to test gas being detected.
Mobile ions diffuse to both surfaces of the oxide where they may be eliminated by reaction with adsorbed species. In the absence of gas species which can be oxidized (such as, for instance, carbon monoxide or hydrogen), the electrochemical potential of the sensor may be determined by the Nernst equation and is proportional to the partial pressure of oxygen in the test gas only. In order to achieve sensitivity to hydrogen with this device, the platinum electrode is co-deposited with gold. Since gold is a substantially less efficient donor of electrons than platinum, oxidation rates are reduced, equilibrium conditions are not achieved and the sensor response is sensitive to the composition of the test gas. The electrochemical potential which develops becomes non-Nernstian, and is a complicated function of the kinetics and mass transfer associated with all species reacting at the electrode.
Just like the CC sensor, the MO sensor has serious disadvantages. The sensor is not hydrogen-specific and all oxidizable gases in the test gas contribute to the sensor signal. The response is relatively slow and it can take up to 20 seconds to reach 50% of maximum signal when exposed to 1% hydrogen in air at flows below 200 standard cubic centimeters per minute (sccm); the recovery time is even slower taking up to 5 minutes to reach 50% of maximum signal when exposed to less than 200 sccm of air. Finally, in order to achieve even these orders of response time, the device must be operated at temperatures above 350xc2x0 C. Operating at such temperatures, just as in case of the MOS sensors, subsequently discussed, is potentially unsafe and may cause ignition and/or explosion.
Yet another type of sensor is the metal oxide-semiconductor (MOS) sensor which is also known and is mentioned, for instance, in the U.S. Pat. No. 6,006,582 to Bhandari, et. al. The MOS sensor element comprises an oxide, typically of iron, zinc, or tin, or a mixture thereof, and is heated to a temperature of between about 150xc2x0 C. and about 350xc2x0 C. Bhandari et. al. reported that oxygen absorbs on the surface of the sensor element to create an equilibrium concentration of oxide ions in the surface layers.
The original resistance of the MOS sensor is first measured. When certain compounds, such as, for instance, CO, or hydrocarbons come in contact with the sensor, they are adsorbed on the surface of the MOS element. This absorption shifts the oxygen equilibrium, causing a detectable increase in conductivity of the MOS material.
MOS hydrogen sensors have a number of operational deficiencies and are, therefore, unsatisfactory in many respects. They require frequent calibration and their response times are too long (up to 3-5 minutes). Bhandari et. al. noted that the MOS sensors are unsafe and can cause ignition and explosion, and are susceptible to being poisoned with halogenated vapors. Like the CC and the MO sensors discussed above, they are not hydrogen specific. All volatile organic compounds as well as gases containing hydrogen will react with the sensor materials in the sensing elements of these detectors, thereby providing false readings.
Still yet another sensor is the catalytic gate (CG) sensor, the simplest embodiment of which is a MOS structure, where the metal is usually platinum or palladium deposited on an insulator, such as silicon dioxide. Hydrogen dissociates on platinum or palladium and subsequently diffuses into the bulk of the metal. Hydrogen atoms which arrive at the metal-insulator interface, form a dipole layer, polarizing the interface and consequently changing its electrical characteristics. The CG sensor also has serious drawbacks, particularly slow response time when the surface is contaminated. The surface of platinum or palladium is very much susceptible to contamination and poisoning.
There exists no known prior art teaching a hydrogen-specific sensor, which is immune to the interference from other gases, which quickly responds and which is not susceptible to poisoning. Yet, as discussed above, such sensor is highly desirable and the need for such sensor, which is also low cost, lightweight and of a miniature size, is acute.
The present invention discloses such sensor. It therefore is an object of the present invention to provide an improved hydrogen sensor and hydrogen sensing methodology overcoming the aforementioned deficiencies of the previously known hydrogen detectors.
The present invention is directed to a hydrogen sensor based upon an AB5-based class of metal hydride materials described below. Materials of this type are capable of reversibly adsorbing hydrogen, first as a so-called xcex1-hydride solid solution, and then as an ordered xcex2-hydride phase. The reactions of hydrogenization or dehydrohenization are very fast and in most applications the speed of these reactions is limited only by heat transfer between particles. Diffusion of hydrogen is controlled and proportional to the square root of pressure of hydrogen, according to Sievert""s law. See, H. Gerard and S. Oho, Hydrogen in Intermetallic Compounds II, Chapter 4, Springer-Verlag, 1992. Thus, the detection of hydrogen is very quick.
This invention takes advantage of a fact that AB5-based metal hydride materials are characterized by large volume expansion when they are absorbing hydrogen. For example, LaNi5 exhibits about 30% lattice expansion by volume (about 8.4% along the xe2x80x9caxe2x80x9d axis and about 8.1% along the xe2x80x9ccxe2x80x9d axis) upon having absorbed the maximum amount of about 1.5% of hydrogen by weight. Crystalline thin films of LaNi5 have been successfully deposited on glass substrates and, upon absorption of hydrogen, exhibited lattice expansion only in a direction perpendicular to the substrate. See, for example, T. Sakai, et. al., J. Electrochem. Soc., 138, 909 (1991); G. Adachi, K. Niki, and J. Shiokawa, J. of Less-Comm. Met., 81, 345 (1981).
This invention proposes to register such expansion, the degree of which is proportional to the amount of hydrogen absorbed, and to measure it by making the deposited AB5 compound an electrode in a capacitive device or a tunneling device, including a MEMS device.
Based on the foregoing, a hydrogen sensor is proposed which has a number of important advantages not available with any other previously known sensor:
(a) the sensor is hydrogen-specific, because only hydrogen penetrates the lattice causing the expansion of the lattice;
(b) the measurement is very quick because the speed of the reactions of hydrogenization and dehydrogenization is extremely high;
(c) carbon monoxide does not poison the sensor if it is maintained at about 115xc2x0 C.; and
(d) oxygen and water do not poison AB5 because the surface of the component A forms a self-limiting oxide/hydroxide layer protecting the sub-surface of the component B and protects the reactivity of the latter towards hydrogen.
See, for example, H. C. Siegmann, L. Schlapbach, and C. R. Brundle, Phys. Rev. Lett., 40, 972 (1978).
A first aspect of the invention provides a device adapted for detecting the presence, and for measuring of, the amount of hydrogen present in an environment, said device comprising a body of a metal hydride hydrogen-absorbent material, said material having a capability to expand upon absorption of said hydrogen, a first electrode, and a dielectric material located between said body of said metal hydride material and said first electrode.
A second aspect of the invention provides a method for detecting the presence, and for measuring, of the amount of hydrogen detected using a sensor, comprising steps of providing a body of a metal hydride hydrogen-absorbent material, said material having a capability to expand upon absorption of said hydrogen, providing a first electrode, providing a dielectric material disposed between said body of said metal hydride material and said electrode, exposing said sensor to an environment which contains or may contain hydrogen, and detecting the presence, and measuring of, the amount of hydrogen contained in said atmosphere using a measuring device capable to measure and register an expansion of said hydrogen-absorbent material upon absorption of said hydrogen.